1. Field of the Invention
The present invention relates in general to an orthogonal television receiver for receiving a composite video signal in which a color signal is interleaved in a luminance signal, and more particularly to an apparatus for separating luminance and color signals from a composite video signal, which is capable of suppressing a cross luminance phenomenon of the luminance signal and a cross color phenomenon of the color signal, the cross luminance phenomenon being a dot interference phenomenon which occurs in the luminance signal, the cross color phenomenon being a rainbow pattern color spread phenomenon which occurs in the case where the composite video signal has a diagonal variation.
2. Description of the Prior Art
To use efficiently a limited frequency band in a general orthogonal modulation color television broadcasting system, a composite video signal in which a color signal is interleaved in a luminance signal is transmitted to an orthogonal color television receiver, which separates the luminance and color signals from the transmitted composite video signal.
The color television receiver may imperfectly separate the luminance and color signals from the composite video signal. This imperfect separation of the luminance and color signals causes dot interference and color spread phenomenons, resulting in a degradation in a picture quality.
In order to remove the dot interference and color spread phenomenons due to the imperfect separation of the luminance and color signals, a conventional NTSC system first extracts the color signal from the composite video signal using a characteristic of the color signal of being 180.degree. out of phase every line and then extracts the luminance signal from the composite video signal by removing the extracted color signal. A conventional apparatus for separating the luminance and color signals from the composite video signal in this manner will hereinafter be described in detail with reference to FIG. 1.
Referring to FIG. 1, there is shown a block diagram of the conventional apparatus for separating the luminance and color signals from the composite video signal. As shown in this drawing, the conventional apparatus comprises a line comb filter 1 for extracting the color signal from the composite video signal CV, a band pass filter 2 for extracting a frequency component around a chrominance subcarrier from the extracted color signal from the line comb filter 1 and outputting the resultant color signal C, a delay 3 for delaying the composite video signal CV so that the composite video signal CV and the color signal C from the band pass filter 2 are in phase, and a subtracter 4 for subtracting the color signal C from the band pass filter 2 from the delayed composite video signal CV from the delay 3 to output the luminance signal Y.
In operation, the color signal is extracted from the composite video signal CV in the line comb filter 1 and then applied in the band pass filter 2. The band pass filter 2 acts to extract the frequency component around the chrominance subcarrier from the extracted color signal from the line comb filter 1 and output the resultant color signal C. The composite video signal CV is also delayed by a predetermined period of time by the delay 3, so that it is in phase with the color signal from the band pass filter 2, and then applied to the subtracter 4, which is also applied with the, color signal C from the band pass filter 2. As a result, the subtracter extracts the luminance signal Y by subtracting the color signal C from the band pass filter 2 from the delayed composite video signal CV from the delay 3.
In the above-mentioned conventional apparatus for separating the luminance and color signals from the composite video signal, however, there is a trade-off between a resolution and a cross-talk resulting from the separated luminance and color signals. Namely, setting widely a band width of the frequency component around the chrominance subcarrier in the extraction of the color signal, the resolution of the color signal is increased, while a probability that a cross color phenomenon of the color signal occurs is increased. Also, in the case where the composite video signal has a diagonal variation or the color signal thereof has a small amplitude, the degradation in the picture quality is greatly influenced by the color spread phenomenon resulting from the cross color phenomenon rather than a reduction in the resolution of the color signal. On the other hand, the degradation in the picture quality is greatly influenced by a reduction in the resolution of the luminance signal rather than a cross luminance phenomenon of the luminance signal.
The line comb filter with a sharp cut-off characteristic is used to remove the cross color phenomenon of the color signal in the separation of the luminance and color signals in the conventional apparatus. However, if the frequency band width of the band pass filter is widely set, there is increased a probability that the color signal interferes with the luminance signal. Also, if the frequency band widths of the line comb filter and the band pass filter are widely set, the resolution of the luminance signal is reduced and the cross color phenomenon of the color signal takes place.
To solve these problems, a luminance/color signals separating apparatus shown in U.S. Pat. No. 4,864,398 comprises 1 horizontal line (H) and 2H comb filters and a band pass filter to extract the color signal from the composite video signal. However, in this apparatus, a dot crawling phenomenon may take place due to an instability of the band pass filter when the composite video signal has a horizontal variation. Also, this apparatus cannot cope with the case where three lines of the composite video signal all have variations. Further, the cross color phenomenon of the color signal occurs greatly when the composite video signal has the diagonal variation.
Another luminance/color signals separating apparatus is shown in U.S. Pat. No. 4,893,176. This apparatus is adapted to extract the luminance and color signals from the composite video signal by varying operating characteristics of 6H and 2H comb filters which are used to extract the color signal. However, in this apparatus, it is difficult to remove the cross luminance phenomenon which takes place when the composite video signal has two or three line variations.